Time delay circuit and relaxation oscillator



April 1942- w. s. MORTLEY 2,279,007

TIME DELAY CIRCUIT AND RELAXATION OSCILLATOR Filed March 1, 1940 INVENTOR W/LFR/D S/NDEN MORTLEY ATTORNEY Patented Apr. 7, 1942 TIME DELAY CIRCUIT AND RELAXATION OSCILLATOR Wilfrid Sinden Mortley, Great Baddow, Chelmsford, England, assignor to Radio Corporation of America, a corporation of Delaware Application March 1, 1940, Serial No. 321,657 In Great Britain March 25, 1939 Claims.

This invention relates to time delay circuit arrangements and to relaxation oscillation generators.

In many cases, such for example as delayed switching on of a high tension circuit for thermionic valves or the control of automatic trafllc light signals, time delays of the order of minutes are required. If it is sought to obtain such a time delay by means of a simple resistance-capacity circuit, a large capacity with very small accidental leakage and a very high value of resistance are required, and in order to ensure constancy of time delay the apparatus must be sealed to avoid leakage paths being set up by the deposition ofdirt or moisture. These requirements make this type of time delay circuit very unsatisfactory in practice for the obtaining of long time delays and accordingly it is common to use thermal time delay devices. These, however, are almost equally unsatisfactory in operation and, moreover, will not give a large time delay in one direc tion and a small time delay in the other. For

example, consider the case of a time delay switch of the thermal type arranged as in known fashion to control the anode supply to a gas filled valve.

If the supply is interrupted for, say, five seconds, and then restored, the thermal time delay switch may not have had time to open, and the restoration of the supply will accordingly probably severely damage, or even destroy the gas filled valve.

The present invention seeks to overcome the above defects and difficulties and to provide a time delay circuit arrangement which will give a long time delay in one direction and a short time delay in the other. By arranging a relay or the like actuated by a time delay circuit in accordance with this invention to control the time delay circuit itself, the said time delay ararngement in accordance with the invention may be transformed into a relaxation oscillation generator.

According to this invention a time delay circuit or relaxation oscillation generator comprises a thermionic vacuum valve circuit to which negative feed back is applied by means including an impedance of predetermined time delay characteristics and the grid-cathode potential of the valve or valve employed is made dependent upon the voltage set up across said impedance.

The accompanying drawing illustrates one embodiment of the invention suitable for the delayed switching of an alternating current or direct current supply, and in which the single figure shows the invent-ion utilized as a relaxation oscillator.

In the accompanying drawing, wherein, for the sake of convenience in description resistances and a condenser will be referred to by the refences used also to indicate their respective values, there is employed a triode V in whose cathode leg is connected (in the order stated) a resistance of value R2 in series with the winding W of a relay or other electromagnetic operating device by which the required switching or other control is to be effected, the end of said winding remote from the cathode being connected to earth or frame E. In shunt across the series connected resistance and relay winding is a condenser of value C in series with a resistance of value RI one terminal of the last mentioned resistance being connected to the cathode. The junction point of the resistance RI with the condenser C is connected to the grid of the valve through a resistance of value R3. A source of high tension voltage HT is provided so that it can be connected between the anode terminal Tl of the valve and earth or frame E. The term ground may here be used to designate any point or surface or fixed alternating current potential or zero radio frequency potential; such a surface may be earth or the frame E.

With this arrangement, if the high tension voltage from source HT be suddenly applied between the anode and earth through a switch S or other suitable device, the current through the relay winding W will be very small at first and will then rise slowly until it reaches the normal current for zero voltage between grid and cathode. Under these conditions the time constant of the circuit is C(A+I)(RI+R2) where A is the amplification of the valve circuit. If R3 is zero, a very short interruption (only a fraction of a second) of the supply will, upon restoration of the supply, cause current to start again at a very low value which graudally increases. If R3 is of large value, an interruption of the high tension of the order of a few seconds will not cause the current, on restoration of the supply, to be lowered much and the relay will thus make contact again immediately. This hold on delay has a time constant given by C(RG+R2 +R3), where Re is the average grid-cathode resistance.

Instead of operating the arrangement by interruption of the high tension supply, it may be restored to the small current" condition by connecting the junction point of RI and R3 to earth, in which case a required restoration delay may be introduced by inserting a resistance in this connection.

The invention is capable of considerable modification in practice, to obtain still longer delay times. For example, a separate valve may be provided for operation of the. control relay, while negative reed back may be applied through an amplifier containing more than one stage.

By employing the control relay to control the time delay circuit itself, e. g., by connecting the relay contacts in the particular embodiment described between the junction point T2 of RI and R8 and earth, it is possible to cause the apparatus to operate as a long period relaxation oscillotion generator. In such a case where the relay contacts are connected as described an anti-sparking resistance is preierably included in the relay contact circuit and it is also advantageous to connect a condenser across the series circuit comprising R2 and the relay winding so that the relay will remain closed for suilicient time to discharge the condenser C lully. In the operation oi the single figure or the drawing, the closure of the contacts 01 relay W will short circuit condenser C, so to speak, and cause the grid to become negative, thus reducing the current through the relay W and causing it to release. Alter a short time, depending upon the time constants o! the circuit, the relay will again operate and the process be repeated.

The relay winding need not be in the cathode leg circuit though it is preferred so to connect it; it is possible, however, to connect the said relay winding in the anode circuit.

In practice the value of RI would be, preferably, much greater than that or R2.

In operation, assuming that the inductance of the relay is negligible, and that the resistance or it is merged in R2, it high tension is suddenly applied between the anode of the valve and earth by switch 8, a small current flows through the valve and the resistance R2, generating across R2 a voltage which is passed via C and R3 to the grid. Thus the voltage on the grid becomes negative in relation to the cathode and is enough almost to prevent the flow of anode current. This voltage appears across C and RI in series. If it remained constant the condenser C would charge exponentially with a time constant CRI, the voltage across RI decreasing at the same rate. Actually, as soon as the negative voltage E (the voltage between grid and cathode) starts to tall, the anode current grows and the charging voltage developed across R2 increases, thus delaying the full charging of the condenser C. Finally the condenser does become fully charged, but with a time constant much greater than CRI, the potential or the grid approaches that of the cathode, and the anode current flowing will be as determined by the value or R2 and the type of valve used. It, now, the high tension supply is disconnected irom valve V, the condenser C discharges through R2, R3 and the grid cathode path of the valve resistance R0. The time constant is C(Ro-i-R2+R3). If new the high tension be again restored to valve V, the condenser C will already be charged by an amount depending upon the time constant C(Ro+R2+R3) and the shortness of the time interruption.

The values or am or the resistances or of the condenser may be made variable to vary the delay as may be desirable.

What is claimed is:

1. A negative feed back time delay circuit comprising a thermionic valve having an anode, a

grid and a cathode, means for applying a positive polarizing potential to said anode relative to said cathode, a relay winding in the anodecathode circuit of said valve, a first resistor having one terminal connected to said cathode and its other terminal connected to ground, a connection including a condenser in series with a second resistor shunting said first resistor, said condenser having one terminal connected to ground, and a third resistor connecting said grid to the junction point of said condenser and said second resistor, the time constants of said resistors and said condenser being such that the sudden application of said positive polarizing potential to said anode by said means causes an initial small current to flow in said valve which gradually charges said condenser and which increases to an amount suiilcient to operate said relay, and the subsequent disconnection of said positive polarizing potential causes a discharge of said condenser through said first and third resistors and through the grid-cathode path oi the valve with a consequent release of said relay.

2. A negative ieed back time delay circuit comprising a thermionic valve having an anode, a grid and a cathode, means for applying a positive polarizing potential to said anode relative to said cathode, a first resistor in series with the winding of a relay and connecting said cathode to ground, a connection including a condenser in series with a second resistor shunting said first resistor, said condenser having one terminal connected to ground, and a third resistor connecting said grid to the junction point of said condenser and second resistor, the time constants of said resistors and said condenser being such that the sudden application of said positive polarizing potential to said anode by said means causes an initial small current to flow in said valve which gradually charges said condenser and which increases to an amount sufiicient to operate said relay, and the subsequent disconnection of said positive polarizing potential causes a discharge of said condenser through said first and third resistors with a consequent release of said relay.

3. A negative feed back time delay circuit comprising a thermionic valve having an anode, a grid and a cathode, means for applying a positive polarizing potential to said anode relative to said cathode, a first resistor in series with the winding of a relay and connecting said cathode to ground, a connection including a condenser in series with a second resistor shunting said first resistor, said condenser having one terminal connected to ground, and a third resistor connecting said grid to the junction point or said condenser and second resistor, a direct connection from said junction point to the armature of said relay, and a direct connection to ground from a contact of said relay adapted to engage said armature, the time constants of said resistors and said condenser being such that the sudden application of said positive polarizing potential to said anode by said means causes an initial small current to flow in said valve which gradually changes said condenser and which increases to an amount sufiicient to operate said relay, whereupon the engagement of said armature and contact causes the condenser to discharge through the gridcathode path of said valve with a subsequent release of said relay.

4. A negative feed back time delay circuit comprising a thermionic valve having an anode, a grid and a cathode, means for applying a positive polarizing potential to said anode relative to said circuit 01' said valve, a first resistor having one terminal connected to said cathode and its other terminal connected to ground, a connection ineluding a condenser in series with a second resistor shunting said first resistor, said condenser having one terminal connected to ground, and a third resistor connecting said grid to the Junction point 01' said condenser and said second resistor, the grid-cathode potential of said valve being dependent upon the voltage set up across 7 said condenser.

5. A negative feed back time delay circuit com- 'prisinga thermionic valve having an anode, a

grid and a cathode, means including a source of potential for applying a positive polarizing potential to said anode relative to said cathode, a first resistor in series with the winding or a relay and connecting said cathode to ground, a connection including a condenser in series with a second re- 20 2,279,0q7 7 3 moat, a relay winding in the anode-cathode sistor shunting said first resistor. said condenser having one terminal connected to ground, and a third resistor connecting saidigrid to the Junction point of said condenser and second resistor, a circuit extending from said junction point to the armature or said relay, and a direct connection between a terminal 01 said source and a contact oi! said relay adapted to engage said armature,

the time constants oi said resistors and said eondenser'being such that the sudden application or said positive polarizing potential to said anode by said means causes an initial s'mallyurrent to new in said valve which gradually said condenser and which increases to an' amount sumcient to operate said relay.- whereupon the engagement or said armature and contact causes the condenser to discharge through the gridcathode path or said valvewith a subsequent release oi said relay. WIIFRID SINDEN MORTLEY. 

